首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   44263篇
  免费   4136篇
  国内免费   33篇
  2022年   242篇
  2021年   603篇
  2020年   337篇
  2019年   412篇
  2018年   556篇
  2017年   512篇
  2016年   926篇
  2015年   1595篇
  2014年   1735篇
  2013年   2205篇
  2012年   2760篇
  2011年   2841篇
  2010年   1860篇
  2009年   1794篇
  2008年   2607篇
  2007年   2621篇
  2006年   2555篇
  2005年   2522篇
  2004年   2457篇
  2003年   2420篇
  2002年   2369篇
  2001年   501篇
  2000年   387篇
  1999年   569篇
  1998年   674篇
  1997年   481篇
  1996年   436篇
  1995年   421篇
  1994年   401篇
  1993年   409篇
  1992年   341篇
  1991年   314篇
  1990年   331篇
  1989年   247篇
  1988年   297篇
  1987年   275篇
  1986年   269篇
  1985年   330篇
  1984年   375篇
  1983年   337篇
  1982年   434篇
  1981年   432篇
  1980年   414篇
  1979年   268篇
  1978年   298篇
  1977年   265篇
  1976年   274篇
  1975年   209篇
  1974年   280篇
  1973年   251篇
排序方式: 共有10000条查询结果,搜索用时 515 毫秒
931.
A novel PGE2 analog (CL 116,069) was shown to be effective in dogs as a nasal decongestant. Threshold doses were approximately 0.1 μg/kg with intravenous administration and between 0.08 and 4 μg with topical administration. CL 116,069 was compared to 17-phenyl-trinor PGE2 (CL 116,147), a compound recently studied in humans, and xylometazoline, a well-known nasal decongestant. When given i.v., efficacious doses of xylometazoline tended to raise blood pressure and be shorter acting than the PGs, which did not affect blood pressure. When given topically, all three were long-acting. CL 116,069 usually had the lowest threshold and CL 116,147 usually induced the smallest response. All three agents were more effective than PGE1 or PGE2. A 30-day (b.i.d., topical) toxicity test with CL 116,069 produced no inflammation or nasal pathology and no loss in tissue sensitivity. Invitro examination of xylometazoline and CL 116,069 for vascoconstrictor activity on dog isolated mucosa revealed a response profile similar to that observed with these agents invivo; i.e., the magnitude of response was comparable for both agents but the t 12 was only 74 minutes for xylometazoline and greater that 6.5 hours for CL 116,069. The data suggest that CL 116,069 may provide a therapeutic alternative in which constriction of the nasal blood vessels need not be associated with a generalized vasoconstrictor liability.  相似文献   
932.
Pollen grains of Haplopappus and related genera in the subtribe Solidaginae from North and South America were examined by light and scanning electron microscopy. The grains are consistently tricolporate and echinate. Some genera can be distinguished by pollen size, spine length, and number of spine rows between colpi. Based on these characters, the divergence of Benitoa from other members of the subtribe, as indicated by its morphology and secondary chemistry, is supported. Additionally, the recently suggested absence of a close relationship between Pyrrocoma and Oonopsis is indicated by their contrasting pollen types. This study demonstrates the potential of pollen studies in distinguishing some taxonomic groups in the Astereae.  相似文献   
933.
934.
935.
Summary The fine structure of the tentacles of the articulate brachiopod Terebratalia transversa has been studied by light and electron microscopy. The epidermis consists of a simple epithelium that is ciliated in frontal and paired latero-frontal or latero-abfrontal longitudinal tracts. Bundles of unsheathed nerve fibers extend longitudinally between the bases of the frontal epidermal cells and appear to end on the connective tissue cylinder; no myoneural junctions were found. The acellular connective tissue cylinder in each tentacle is composed of orthogonal arrays of collagen fibrils embedded in an amorphous matrix. Baffles of parallel crimped collagen fibrils traverse the connective tissue cylinder in regions where it buckles during flexion of the tentacle.The tentacular peritoneum consists of four cell types: 1) common peritoneal cells that line the lateral walls of the coelomic canal, 2) striated and 3) smooth myoepithelial cells that extend along the frontal and abfrontal sides of the coelomic canal, and 4) squamous smooth myoepithelial cells that comprise the tentacular blood channel.Experimental manipulations of a tentacle indicate that its movements are effected by the interaction of the tentacular contractile apparatus and the resilience of the supportive connective tissue cylinder. The frontal contractile bundle is composed of a central group of striated fibers and two lateral groups of smooth fibers which function to flex the tentacle and to hold it down, respectively. The small abfrontal group of smooth myoepithelial cells effects the re-extension of the tentacle, in conjunction with the passive resiliency of the connective tissue cylinder and the concomitant relaxation of the frontal contractile bundle.The authors wish to express their appreciation to Professor Robert L. Fernald for his advice and encouragement throughout the course of this study. Some of the work was conducted at the Friday Harbor Laboratories of the University of Washington. The authors are indebted to the Director, Professor A.O.D. Willows, for use of the facilities. Part of this study was supported by NIH Developmental Biology Training Grant No. 5-T01-HD00266 and NSF grant BMS 7507689  相似文献   
936.
937.
938.
Summary Blood samples from 509 Macushi and 623 Wapishana Amerindians of Northern Brazil and Southern Guyana have been analyzed with reference to the occurrence of rare variants and genetic polymorphisms of the following 25 systems: (i) Erythrocyte enzymes: acid phosphatase-1, adenosine deaminase, adenylate kinase-k, carbonic anhydrase-1, carbonic anhydrase-2, esterase A1,2,3, esterase D, galactose-1-phosphate uridyltransferase, isocitrate dehydrogenase, lactate dehydrogenase, malate dehydrogenase, nucleoside phosphorylase, peptidase A, peptidase B, phosphoglucomutase 1, phosphoglucomutase 2, phosphogluconate dehydrogenase, phosphohexoseisomerase, triosephosphate isomerase and (ii) Serum proteins: albumin, ceruloplasmin, haptoglobin, hemoglobin A, hemoglobin A2 and transferrin. Fifteen different rare variants were detected, involving 11 of these systems. In addition, a previously undescribed variant of ESA1,2,3 which achieves polymorphic proportions in both these tribes is described. Excluding this variant, the frequency of rare variants is 1.1/1000 in 12510 determinations in the Macushi and 4.7/1000 in 15 396 determinations in the Wapishana. The ESA1,2,3, polymorphism was not observed in 382 Makiritare, 232 Yanomama, 146 Piaroa, 404 Cayapo, 190 Kraho and 112 Moro. Irregularities in the intratribal distribution of this polymorphism in the Macushi and Wapishana render a decision as to the tribe of origin impossible at present. Gene frequencies are also given for previosly described polymorphisms of 5 systems: haptoglobin, phosphoglucomutase 1, erythrocyte acid phosphatase, esterase D, and galactose-1-phosphate-uridyl-transferase.Research supported by the National Science Foundation and the Energy Research and Development Administration.  相似文献   
939.
940.
A viable, male progeny resulted from a two-year captive pairing of a male red spider monkey (A. g. panamensis) and a female black spider monkey (A. f. robustus). While the pelage colorations of the parental species are distinctive, the offspring was intermediate in appearance during maturation. Such hybridization apparently occurs naturally in Panama; two feral spider monkeys, captured from an area of sympatry in the Province of Panama east of the Canal Zone, were characteristic of the laboratory cross.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号